This invention relates generally to a process for extracting technetium from nuclear wastes and more particularly to a process for extracting technetium from alkaline waste solutions containing technetium and high concentrations of alkali metal nitrates. The United States has rights to this invention pursuant to Contract No. DE-ACO5-840R21400 with Martin Marietta Energy Systems, Inc., awarded by the U.S. Department of Energy.
Fission products resulting from the irradiation of nuclear reactor fuels require long-term storage in an insoluble form. Approximately 1.8 metric tons of technetium is contained in 60 million gallons of high-level radioactive wastes stored at Hanford, Wash. These wastes are highly alkaline (pH.gtoreq.13) and are highly concentrated in alkali metal nitrates, such as sodium nitrate. Removal of technetium from such wastes is warranted because of safety considerations in waste processing and environmental risks associated with long-term storage of the waste. In addition, these wastes provide a resource from which technetium can be recovered for experimental uses and other applications.
Several processes have been developed to extract technetium from nuclear waste solutions which are strongly acidic. One process focused on recovering palladium and technetium values by contacting the acidic waste solution with an extractant of tricaprylmethylammonium nitrate (sold under the trademark Aliquat.RTM. 336, Henkel Corporation, Tucson, Ariz.) in an inert hydrocarbon diluent to extract palladium and technetium from the waste solution. The palladium and technetium values were then recovered from the extractant with a strong nitric acid strip solution. Although this method is useful with acidic solutions, its application to the recovery of technetium from alkaline waste solutions requires stripping by means of nitric acid, which is not particularly safe nor economical.
Another method used cyclohexanone to extract technetium from alkaline media. This process is limited, however, in that cyclohexanone is somewhat toxic, fairly volatile (BP 155.6.degree. C., flashpoint 46.degree. C.), and significantly water soluble (50 grams per liter at 30.degree. C.), thus making this diluent less suitable for use in extracting technetium. Other disadvantages of cyclohexanone include appreciable co-extraction of nitrate salts and known emulsion problems in stripping.
Other techniques have used macrocyclic polyethers ("crown ethers") to extract technetium from strongly acidic waste solutions. One method used a crown ether in a water-insoluble diluent to extract strontium and technetium from nitric acid waste solutions which were up to 6 molar in nitric acid. A related method involved separating strontium and technetium from a waste solution by first preparing a 2-6 M nitric acid sample solution comprising dissolved strontium and technetium in a waste solution and then passing the nitric acid sample solution through a chromatographic separation column comprising a solution of a crown ether in a water-insoluble diluent. Because these methods require a nitric acid solution or call for pretreatment of the solution with nitric acid, they neither describe nor suggest that these techniques may be applied to extracting technetium from alkaline solutions highly concentrated in alkali metal nitrates.
Some work has dealt with the use of crown ethers to extract technetium under alkaline conditions. One study examined the extraction of technetium from alkaline media using dibenzo-18-crown-6 and discussed the effects of crown cavity size, substitutions, and type of base on the distribution coefficient of technetium. Another study investigated the extraction of technetium by solutions of dicyclohexyl-18-crown-6 in trichlorobenzene as a function of nitric acid, potassium hydroxide, and crown ether concentrations. Neither of these studies, however, addressed how high concentrations of alkali metal nitrates such as sodium nitrate in the alkaline media would affect the distribution coefficient of technetium. As will be discussed later, it has been found that the concentration of sodium nitrate in the alkaline waste solution plays a key role in determining the efficiency of extraction and stripping.
Accordingly, a need in the art exists for a method of extracting technetium from alkaline waste solutions that does not require acidification of the media, uses suitable diluents, and works efficiently in the presence of high concentrations of alkali metal nitrates.